Regions with open inter-state borders are not limited to the Schengen Area. Even within unions like the union state of Russia and Belarus, there are regions with open borders, despite being part of two countries. However, the number of countries involved is still lower compared to Schengen.
Another notable union with open borders is MERCOSUR, spanning a significant part of South America and involving numerous countries in the region, including associate members and observers. This union essentially covers almost all of Latin America, making it a key player in the region's economic and political landscape.